Geological Structure of the Neogene Formations in the Middle Part of Ishikari Plane (Sapporo Lowland) in Hokkaido, Japan (Part 1)

Abstract

The middle part of Ishikari Plane (or Sapporo Lowland) is situated in Lat. 40°00'-43°25' N. and Long. 141°12'-141°53' E. In this plane, there are the Neogene Tertiary formations which are overlaid with the Quaternary formations of about 100m in thick. 1. The Neogene Tertiary formations cropping out in the surrounding mountains district are overlaid with unconformity on the Paleogene coal-bearing formations, the Cretaceous System, and so-called “Pale-ozoic” formations. 2. In the Neogene formations of this plane, according to the stage of depositon, there are marked variations in the density of deposits and distribution of fossils. There is no remarkable unconformity, however, in the Neogene Tertiary formations and each formation distributes in all the area, but the thickness of formations is changeable and most thick between the Nopporo Hill and Umaoi Hill. 3. On the land surface or near the surface of this plane, igneous rocks have not been discovered and will not be discovered in future. 4. The Neogene formations have folded structures with axes of approximately north-south direction and they are thought to have been resulted from upheavals of the Hokkaido Backbone Mountain Lands or Hidaka Mts. 5. While the synclinal parts of folding structures are large and broad, their anticlinal parts make steep dip wings and narrow forms. Anticlinal parts are assymmetrical forms and thir axial planes are inclined to west ; that is, they show the nature of “west steep-east slow.” 6. The axial planes of anticlinal structures in the eastern area are extreamly dipping. The angle of dip is smaller in the western area and it takes almost the similar shape of symmetrical anticline structure in the Ishikari coastal region. On the contrary, in the northern mountainous districts of the plane, the axial planes are inclined to east and the anticlinal structures show “east steep-west slow” character. In these mountain regions where crops out of basement complex as Kobato Paleozoic sediments, folding waves are pushed back for eastern side against the lateral forces caused by the upheaval of backbone mountains. Consequently, these folding waves advanced towards the Neogene basin axis (10) (Tomakomai-Takikawa Line) from both sides of east and west mountains (Fig. 12). 7. In the plane, each anticline structure is continuous without faults or echelon arrangement, and there are some upheavals making dome-like structures. On the contaary, however, there are some sinking areas making structural depressions along the synclinal axes. These depressons are distributed at regular intervals, and the distribution of the recent rivers are related to these depressions. 8. The meander zones of rivers distribute centering around Masarikappu near Ishikari Coast elliptically and radially. This distribution shows that the southern mountains slow upheave contrary to the northern mountains (Kobato Mts.). 9. Accordingly, it is revealed that each basement complex (pre-Neogene) of mountains in the northern, southern, and eastern parts of the plane belongs to different geologic structural unit. It is thought that these three blocks made different block movements and these geological movements produced effects on the Neogene Tertiary structure in this plane. The above are interpreted as follow. (1) Southern mountains to which the Ezo-Fuji Volcanics belong are a range from the Central Mountains in North-East Japan where is the Nasu Volcanic Zone. (2) The eastern mountain range is not connected with the land of North-East Japan. (3) The Abukuma and Kitagami ranges in North-East Japan belong to tne same mountain series with Kobato range in the north of Sapporo Lowland. (4) There are geological structure lines between Sapporo Lowland and mountainous districts.